1. Field of the Invention
The present invention relates to sensing formation properties and other conditions while drilling boreholes into the earth. Specifically, the invention provides for measuring these properties in the very severe environment adjacent to the drill bit using sensors that are inexpensive but have calibration drift. These sensors are re-calibrated by a second set of sensors with high-accuracy located in a less severe environment.
2. Description of the Related Art
The present invention applies to the calibration of sensors within a rotary drilling system using measurements generated by a remote measuring while drilling (MWD) tool. Previously, sensors were located far from the drill bit due to packaging size constraints and to minimize damage and calibration drift caused by the severe accelerations and hostile environment adjacent to the drill bit. This places the sensors a considerable distance from the most desirable location to acquire data, the region immediately adjacent to the bit. The presence of the drilling fluid and the pressure difference between the drilling fluid and the fluids in the formation often compromise the readings of these remotely located sensors. Also, many properties which are valuable when measured near the drill bit, including inclination, azimuth, vibration, rotational position, temperature, etc., cannot be accurately measured remotely. The expense, fragility, and physical size of accurate sensors make it at best impractical, and often impossible for them to be packaged into a small location close to (or within) the drill bit. The less expensive and/or smaller sensors that may be used in the drill string adjacent to the drill bit do not have the desired adequate long-term stability and accuracy.